Found due to this: https://github.com/pytest-dev/py/issues/103

I'm going to submit a PR.

The following is an example of the problem, right?

    >>> fd = os.open('stdout.txt', os.O_CREAT | os.O_WRONLY)
    >>> os.dup2(fd, 1)
    >>> print('spam')
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    OSError: [WinError 87] The parameter is incorrect

Code like this used to work, so possibly this needs to be addressed more generally. I have a couple ideas, but they're major changes. It's simpler to require code that pulls the rug out from under _WindowsConsoleIO to rebind sys.std*. 

For a counterexample, PyOS_StdioReadline always checks for a console and falls back on the old my_fgets function for a non-console. For example:

    >>> open('stdin.txt', 'w').write('x=42; os.dup2(oldfd, 0)\n')
    24
    >>> fd = os.open('stdin.txt', os.O_RDONLY)
    >>> oldfd = os.dup(0); os.dup2(fd, 0)
    >>> Breakpoint 0 hit
    python36!my_fgets:
    00000000`66e98318 488bc4          mov     rax,rsp
    0:000> g
    >>> x
    42

We might want to open a separate issue for the FD redirection issue since this one was really about the _io._WindowsConsoleIO.write GetLastError() issue.

Also see https://github.com/pytest-dev/pytest/pull/2462 where I discuss this more.

Still not sure why it's random in Pytest. I can only guess it's due to another console handle getting opened with the same number or something.

New changeset 523776c3419f6795e78173d53c10e35ec4eed48d by Steve Dower (Segev Finer) in branch 'master':
bpo-30544: _io._WindowsConsoleIO.write raises the wrong error when WriteConsoleW fails (#1912)
https://github.com/python/cpython/commit/523776c3419f6795e78173d53c10e35ec4eed48d

Yeah, make it a separate issue.

This PR fixes an obvious error that we should just fix, so I've merged it. (I might get to the backport today, but if someone else submits it then I'll merge.)

New changeset c63ae1122f84d4188ffadfd1454902093eb10be1 by Steve Dower in branch '3.6':
bpo-30544: _io._WindowsConsoleIO.write raises the wrong error when WriteConsoleW fails (#1912) (#1925)
https://github.com/python/cpython/commit/c63ae1122f84d4188ffadfd1454902093eb10be1